Close matplotlib figure windows cleanly

See qutip/qutip#1648

Co-Authored-By: Simon Cross <165551+hodgestar@users.noreply.github.com>
Co-Authored-By: Drew Parsons <26508288+drew-parsons@users.noreply.github.com>

tests/test_processor.py

@@ -170,8 +170,10 @@ def testPlot(self):
         processor.add_control(sigmaz())
         processor.pulses[0].tlist = tlist
         processor.pulses[0].coeff = np.array([np.sin(t) for t in tlist])
-        processor.plot_pulses()
-        plt.clf()
+        fig, _ = processor.plot_pulses()
+        # testing under Xvfb with pytest-xvfb complains if figure windows are
+        # left open, so we politely close it:
+        plt.close(fig)
 
         # cubic spline
         tlist = np.linspace(0., 2*np.pi, 20)
@@ -180,7 +182,10 @@ def testPlot(self):
         processor.pulses[0].tlist = tlist
         processor.pulses[0].coeff = np.array([np.sin(t) for t in tlist])
         processor.plot_pulses()
-        plt.clf()
+        fig, _ = processor.plot_pulses()
+        # testing under Xvfb with pytest-xvfb complains if figure windows are
+        # left open, so we politely close it:
+        plt.close(fig)
 
     def testSpline(self):
         """